Loans and Allowance for Credit Losses - Geographic and Collateral Concentration (Details)	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Geographical concentration
Concentration risk
Percentage of loan	100.00%	100.00%
Geographical concentration | California
Concentration risk
Percentage of loan	19.20%	18.10%
Geographical concentration | Texas
Concentration risk
Percentage of loan	14.30%	14.20%
Geographical concentration | New York
Concentration risk
Percentage of loan	10.50%	9.80%
Geographical concentration | Florida
Concentration risk
Percentage of loan	7.40%	7.80%
Geographical concentration | Illinois
Concentration risk
Percentage of loan	6.00%	5.20%
Geographical concentration | Georgia
Concentration risk
Percentage of loan	6.30%	4.90%
Geographical concentration | North Carolina
Concentration risk
Percentage of loan	3.10%	3.10%
Geographical concentration | Arizona
Concentration risk
Percentage of loan	3.10%	2.80%
Geographical concentration | Washington
Concentration risk
Percentage of loan	2.70%	3.10%
Geographical concentration | Colorado
Concentration risk
Percentage of loan	2.60%	2.80%
Geographical concentration | Other
Concentration risk
Percentage of loan	24.80%	28.20%
Collateral concentration
Concentration risk
Percentage of loan	100.00%	100.00%
Percentage of SBA loan	100.00%	100.00%
Collateral concentration | Lodging
Concentration risk
Percentage of SBA loan	19.30%	17.20%
Collateral concentration | Offices of Physicians
Concentration risk
Percentage of SBA loan	12.90%	12.00%
Collateral concentration | Child Day Care Services
Concentration risk
Percentage of SBA loan	8.00%	7.20%
Collateral concentration | Eating Places
Concentration risk
Percentage of SBA loan	5.60%	5.30%
Collateral concentration | Gasoline Service Stations
Concentration risk
Percentage of SBA loan	3.90%	3.40%
Collateral concentration | Veterinarians
Concentration risk
Percentage of SBA loan	3.40%	3.30%
Collateral concentration | Funeral Service and Crematories
Concentration risk
Percentage of SBA loan	2.00%	1.80%
Collateral concentration | Grocery Stores
Concentration risk
Percentage of SBA loan	2.00%	1.70%
Collateral concentration | Car Washes
Concentration risk
Percentage of SBA loan	1.60%	1.40%
Collateral concentration | Couriers
Concentration risk
Percentage of SBA loan	1.10%	1.00%
Collateral concentration | Other
Concentration risk
Percentage of SBA loan	40.20%	45.70%
Collateral concentration | Multi-family
Concentration risk
Percentage of loan	31.40%	23.80%
Collateral concentration | SBA
Concentration risk
Percentage of loan	13.80%	17.40%
Collateral concentration | Retail
Concentration risk
Percentage of loan	15.70%	17.30%
Collateral concentration | Office
Concentration risk
Percentage of loan	12.80%	13.10%
Collateral concentration | Mixed Use
Concentration risk
Percentage of loan	11.90%	12.90%
Collateral concentration | Industrial
Concentration risk
Percentage of loan	6.70%	7.10%
Collateral concentration | Lodging/Residential
Concentration risk
Percentage of loan	2.80%	3.20%
Collateral concentration | Other
Concentration risk
Percentage of loan	4.90%	5.20%